|Hydrocotyle ranunculoides (Apiaceae – floating (marsh) pennywort)
Why The EPPO Panel on Invasive Alien Species made a first categorization of aquatic invasive plants and considered that Hydrocotyle ranunculoides should be added to the EPPO Alert List.
Description H. ranunculoides is a stoloniferous, perennial, aquatic plant, with floating and emergent leaves. It is rooting in the shallow margins of slow flowing waters. It has round-reniform leaves (diameter 2-6 cm), with 3-7 lobes (lobe divisions extend to about mid-leaf). Long petioles (5-35 cm) are attached to leaf edge (although they appear to be attached to the centre of the leaf). It has small white, greenish or yellow flowers (umbels) arising from the leaf base (each flower has 5 tiny petals). In North America, it flowers during July and October. Fruit is 1-3 mm long, elliptic to round, and flattened with faint ribs. Many fibrous roots emerge from the stems at the nodes. In Europe, can be confused with the native H. vulgaris.
Pictures can be viewed on Internet:
Where H. ranunculoides is a native of North America, and has become naturalized in Central and South America.
EPPO region: Netherlands (first found in waterways in summer 1995, it is suspected that it escaped from garden ponds. Since then, its range rapidly expanded, and it is now considered that it will be very difficult to stop its spread), United Kingdom (first recorded as naturalised in the south east in the 1980s. Introduced by the aquatic nursery trade for tropical aquaria and garden ponds, by 2001 it was recorded at 71 sites, mainly in the south of England and south Wales, but it expands in northwest England). It is reported to occur in Portugal and Italy. In the Netherlands, it is now prohibited to sell and possess H. ranunculoides.
North America: Canada, USA (Alabama, Arizona, Arkansas, California, Delaware, Florida, Georgia, Illinois, Kansas, Louisiana, Maryland, Mississippi, New Jersey, New York, North Carolina, Ohio, Oklahoma, Oregon, Pennsylvania, South Carolina, Tennessee, Texas, Virginia, Washington, West Virginia). In some States (Illinois, New Jersey, New York) it is considered as an endangered species.
South and Central America: Argentina, Bolivia , Brazil, Chile, Costa Rica. It is probably present in other countries but data is lacking.
Oceania: Australia (Western Australia). First found in 1983, and measures are taken to eradicate it and prevent any further spread.
Habitat H. ranunculoides can be found in slow-flowing water bodies, particularly ditches, canals, lakes and ponds. Growth is more abundant on eutrophic sites with high organic matter availability.
Damage H. ranunculoides forms dense interwoven mats of vegetation which can quickly cover the water surface. Plants die back in winter, but shoots and roots survive in the bank and wet ground, and then quickly grow into new plants in spring. In UK conditions, mats of vegetation have been observed to grown up to 15 m from the bank in a single season (so approximately 20 cm per day). Leaf matter extends up to 40 cm above the water surface and the interwoven mat of roots and stems can sink up to 50 cm into the water. H. ranunculoides outcompetes other plants species. Its dense mats of vegetation pose a problem for recreational users and may have a negative impact on the ecosystem (lower light penetration to the water, oxygen shortage, higher risks of flooding).
Dispersal Local dispersal is ensured by rooting at nodes, root and stem fragments, and seeds. Fragments of plants can be transported with flowing water to new sites. Waterfowl are also able to spread plant fragments. Over long distance, trade of plants for aquaria and garden ponds can disseminate H. ranunculoides.
Pathway Plants for planting of H. ranunculoides, (soil/water containing viable plant fragments or seeds?).
Possible risks Control is very difficult. Several methods can be used but none is fully effective. As H. ranunculoides forms very thick mats, it is usually recommended to remove plants mechanically before applying herbicides (diquat, 2,4-D amine, MCPA). Shading with dark material may be a possibility, as well as netting to reduce spread. At least in UK and the Netherlands, H. ranunculoides has shown a high potential for invasiveness. Further spread of this species should be avoided.
Source(s) Baas, W.J.; Duistermaat, L.H (1999) The invasion of floating pennywort (Hydrocotyle ranunculoides L.f.) in the Netherlands, 1996-1998. Gorteria, 25(4), 77-82.
Chikwenhere, G.P.; Julien, M.H. (ed.); Hill, M.P. (ed.); Center, T.D. (ed.); Ding-Jianqing (2001) Current strategies for the management of water hyacinth on the Manyame River System in Zimbabwe. Biological and integrated control of water hyacinth: Eichhornia crassipes. Proceedings of the Second Meeting of the Global Working Group for the Biological and Integrated Control of Water Hyacinth, Beijing, China, 9-12 October 2000, 105-108.
Ferreira, A.G.; Souto, C.F.M.; Goidanich, V.; Lipp J.K.; Del Pino, R. (1981) Density, position and dimensions of stomata of plants occurring at Taim Ecological Station, RS, Brazil. Anais do 32 Congresso Nacional de Botanica, Teresina, Brazil, January 1981, 229-232.
Krabben, K.P.M. van der, Rotteveel, A.J.M. (2003) Draft Report of a pest risk assessment of Hydrocotyle ranunculoides. Unpublished document.
Lallana, V.H.; Barrett, P.R.F. (ed.); Greaves, M.P. (ed.); Murphy, K.J. (ed.); Pieterse, A.H. (ed.); Wade, P.M. (ed.); Wallsten, M. (1990) Dispersal units in aquatic environments of the middle Parana River and its tributary, the Saladillo River. Proceedings of the 8th international symposium on aquatic weeds, Uppsala, Sweden, 13-17 August 1990, 151-159.
Checklist of dicotyledonous species associated with Bolivian wetlands by Dr Nur Ritter, University of Fresno (US). http://www.botanize.com/bol_checklist/dicots.html
Invasive Non-Native Species in the UK. University of Liverpool. The Invasive Alien Species Project. Fact Sheet: 2. Invasive Alien Aquatic Plant Species. Hydrocotyle ranunculoides (L.f.), Floating Pennywort by Dr Jon Huckle, 19th February 2002.
Instituto Nacional de Biodiversidad (Costa Rica). Lista de especímenes de Hydrocotyle ranunculoides. http://www.inbio.ac.cr/bims/k03/p13/c045/o0135/f01665/g006947/s019969.htm
Palma, B.; San Martín, C.; Rosales, M.; Zúñiga, Ramírez, C. (1986) Distribución espacial de la flora y vegetación acuática y palustre del estero marga-marga en Chile central. http://biblioweb.dgsca.unam.mx/cienciasdelmar/instituto/1987-2/articulo269.html
USDA Natural Resources Conservation Service. Plant Profile. Hydrocotyle ranunculoides. http://plants.usda.gov
Wageningen University. Environmental Science. Aquatic Ecology and Water Quality Management Group. Invasion of Dutch waters by floating pennywort (Hydrocotyle ranunculoides) (‘Grote Waternavel’). http://www.dow.wau.nl/aew/Projects/info_waternavel.html
EPPO RS 2004/042
Panel review date 2005-03-- Entry date 2004-04